1. Field of the Invention
The present invention relates to lasers, optical amplifiers with the properties of low noise and high gain, and amplification methods.
2. Description of the Related Art
In recent years, the development of an optical amplifier, in which an optical fiber having a core doped with a rare earth element is provided as an amplification medium, has been worked on for applications in the field of optical communication. Particularly, an erbium (Er.sup.3+)-doped fiber amplifier (EDFA) has been developed, and development efforts are being made to increase applications of the EDFA to an optical communication system.
Recently, a wavelength division multiplexing (WDM) technique has been studied extensively to cope with the diversification of communication service to be expected in coming years. The WDM technique is an optical communication technique that uses a system of multiplexing wavelengths for the sake of an effective use of available transmission medium leading to enlarge a transmission volume. One of the characteristics required in the EDFA applied in the WDM technique is a small variation in an amplification gain with respect to a signal wavelength. There are power differentials among optical signals which are transitionally amplified by passing through a multi-stage arrangement of the EDFAs, so that it is difficult to perform the signal transmission with uniform characteristics maintained across all of the wavelengths being used. Presently, therefore, the EDFA showing a flat gain region with respect to the predetermined wavelengths has been investigated by persons skilled in the art.
Attention is being given to an erbium(E.sup.3+)-doped fluoride fiber amplifier (F-EDFA) as a most promising candidate as the EDFA, in which a fluoride-based fiber is used as a host of Er.sup.3 +. The F-EDFA is characterized by its emission spectrum caused by a transition from the .sup.4 I.sub.13/2 level to the .sup.4 I.sub.15/2 level of Er.sup.3+ ions in the fluoride glass at a wavelength band of 1.55 .mu.m.
FIG. 1 shows a typical amplitude spontaneous emission (ASE) spectrum of the F-EDFA. This figure also shows the ASE spectrum of an Er.sup.3 +-doped silica glass fiber (S-EDFA). As shown in the figure, the emission spectrum (a full line in the figure) of the F-EDFA is broader than the emission spectrum (a dashed line in the figure) of the S-EDFA. In addition, the response curve of the F-EDFA is smoother than that of the S-EDFA and is flat on top without any steep portion depended on a wavelength in the predetermined wavelength region (M. Yamada et al., IEEE Photon. Technol. Lett., vol. 8, pp882-884, 1996). Furthermore, experiments of wavelength division multiplexing have been carried out using multi-staged F-EDFAs, for example a cascade configuration with a 980 nm pumped S-EDFA and a 1480 nm pumped F-EDFA (M. Yamada et al., IEEE Photon. Technol. Lett., vol. 8, pp620-622, 1996).
In spite of the above development efforts, the F-EDFA has a problem that it cannot reduce a noise figure (NF) as much as that observed in the S-EDFA because of the following reasons.
FIG. 2 is an energy diagram of Er.sup.3 +. A phonon energy takes a value on the order of 1,100 cm.sup.-1 when the EDFA uses a silica optical fiber as an amplification medium (i.e., in the case of the S-FDFA), so that a favorable population inversion between the .sup.4 I.sub.13/2 level and the .sup.4 I.sub.15/2 level can be formed by an efficient excitation to the .sup.4 I.sub.13/2 level as a result of a phonon emitted relaxation from higher energy levels to the .sup.4 I.sub.13/2 level after exciting to the .sup.4 I.sub.13/2 level by 0.98 .mu.m pump light (FIG. 2 (A)). Consequently, the S-EDFA enables a reduction in the NF to about 4 dB which is close to a quantum limit (3 dB). On the other hand, the F-EDFA cannot perform an excitation to the .sup.4 I.sub.13/2 level using a transmission from the .sup.4 I.sub.15/2 level to the .sup.4 I.sub.11/2 level because of its low phonon energy. That is, the F-EDFA has a phonon energy of about 500 cm.sup.-1 which is almost half of the S-EDFA's phonon energy, so that it is difficult to cause a phonon emitted relaxation from the .sup.4 I.sub.11/2 level to the .sup.4 I.sub.13/2 level and to obtain an amplification gain by 0.98 .mu.m pump light. In this case, therefore, an amplification gain at a wavelength of 1.55 .mu.m is obtained by directly exciting from the .sup.4 I.sub.15/2 level to the .sup.4 I.sub.13/2 level using light at a pump wavelength of about 1.48 .mu.m (FIG. 2 (B)). However, this kind of the excitation is an initial excitation of the gland energy level to the higher energy level, so that it is difficult to make a favorable population inversion in which the number of Er.sup.3+ ions at higher energy levels exceed those at lower energy levels, resulting in the high NF (i.e., 6 to 7 dB).
Therefore, the conventional F-EFDA with favorable noise characteristics has not been realized, compared with that of the S-EFDA.